leetcode No258. Add Digits

Question：

Given a non-negative integer num, repeatedly add all its digits until the result has only one digit.

For example:

Given num = 38, the process is like: 3 + 8 = 11, 1 + 1 = 2. Since 2 has only one digit, return it.

算法：

（1）首先是得到每位数字之和，用while语句实现（内循环）

（2）判断每位数字之和是否是个位数，若不是则继续计算每位数字之和，while语句实现（外循环）

class Solution {
public:
    int addDigits(int num) {
       int add;
       while( num > 9 )
       {
           add = 0;
           while(num > 9)
           {
               add = add +num%10;
               num = num/10;
           }
           num = add + num;
       }
     return num;
    }
};